1. Field of the Invention
The invention relates to an electric device; and more particularly, to a rotating dynamoelectric machine capable of operating at high commutating frequencies, with high efficiency and high power and torque densities.
2. Description of the Prior Art
The electric motor and generator industry is continuously searching for ways to provide motors and generators with increased efficiencies and power densities. The power of an electromagnetic device is related to the exciting frequency of the device (also sometimes known as the commutating or electrical frequency), such that an increase in the exciting frequency of the device increases the power. Thus, machines with higher exciting frequencies are often desired when increased power is desired. The synchronous frequency of a synchronous electric machine can be generally expressed as f=N·P/2, where f is the exciting frequency of the machine in Hz, N is the speed in revolutions per second, and P is the pole count of the machine. From this, it is seen that as the speed of the machine increases, the frequency increases, and the power increases. Likewise, as the pole count increases, the exciting frequency required to attain the same rotational speed increases. However, as the pole count increases, the time rate of change in the magnetic flux in the components of the machine for a given rotational speed also increases, leading to production of waste heat from increased core loss. A substantial fraction of the internally generated heat in conventional devices is core loss, arising principally from hysteresis in the soft magnetic material used in the stator, although magnetic losses also occur in the rotor magnets and other conductive elements exposed to a changing magnetic field.
Past attempts to manufacture high frequency electric machines (i.e., electric machines with a frequency greater than 400 Hz) typically involved low pole counts at high speeds to keep losses within acceptable limits. The vast majority of today's machines use conventional silicon-iron alloy (Si—Fe) containing about 3½% or less by weight of silicon. In particular, losses resulting from the changing magnetic fields at frequencies greater than about 400 Hz in conventional Si—Fe-based materials cause the material to heat to the point where the device cannot be cooled by any acceptable means. Machines using high frequency excitation to attain high power have thus been regarded heretofore as virtually impossible to construct and hence not commercially viable. Nevertheless, there remains a continuing desire for dynamoelectric machines that operate at high exciting frequencies, while yet providing a combination of high efficiency and high power density without any need for elaborate cooling schemes.
The development of amorphous metals and other advanced magnetic materials has caused many to believe that motors and generators made with magnetic cores of these materials potentially could provide substantially higher efficiencies and power densities than those available in conventional motors and generators. In particular, amorphous metals exhibit promising low-loss characteristics, suggesting that a stator made with a magnetic core of amorphous metal theoretically might result in an electric machine with increased efficiencies. However, previous attempts at incorporating amorphous material into conventional machines have not been commercially successful, because most simply involved substituting amorphous material for the silicon-iron in conventional magnetic cores of lower frequency electric machines. While some of these electric machines provide modestly increased efficiencies and lower losses, the lower saturation induction (flux density) of the amorphous metal deleteriously reduces power output. Moreover, the unique mechanical properties of amorphous metal make it significantly more difficult, if not impossible, to process using the techniques ordinarily employed in constructing conventional machines. Based on the unacceptably high handling and forming costs that would be incurred, it thus has not been deemed feasible to make this replacement.
For example, U.S. Pat. No. 4,578,610 discloses a highly efficient motor having a stator constructed by simply coiling a strip of amorphous metal tape, wherein the amorphous strip is wound and then slotted and a suitable stator winding is then placed within the slots.
U.S. Pat. No. 4,187,441 discloses a high power-density machine having spirally wound laminated magnetic cores made from amorphous metal ribbon having slots for receiving stator windings. The patent further discloses using a laser beam for cutting the slots into the amorphous core.
Notwithstanding significant study surrounding the use of amorphous metals in electric machines, to date it has proven very difficult to cost effectively provide a readily manufacturable electric device, which takes advantage of low loss materials. Many have abandoned attempts to develop a commercially viable electric machine having a magnetic core of amorphous metal. Thus it would be desirable to provide a highly efficient electric device, which takes full advantage of the specific characteristics associated with low loss material, thus eliminating the disadvantages associated with the prior art.